Conventionally, as the power plant of this kind, one disclosed in Patent Literature 1 is known. This power plant is for driving left and right drive wheels of a vehicle, and is equipped with an internal combustion engine as power source, and a transmission connected to the engine and the drive wheels. The transmission includes first and second planetary gear units of a general single pinion type, and first and second rotating machines each having a rotor and a stator.
As shown in FIG. 51, the first planetary gear unit has a first ring gear, a first carrier and a first sun gear mechanically connected to the engine, a second carrier of the second planetary gear unit, and the first rotating machine, respectively. The second planetary gear unit has a second sun gear, a second carrier and a second ring gear mechanically connected to the second rotating machine, the drive wheels, and the first rotating machine, respectively. Further, the first and second rotating machines are electrically connected to each other via a controller. It should be noted that in FIG. 51, mechanical connections between elements are indicated by solid lines, and electrical connections therebetween are indicated by one-dot chain lines. Further, flows of power and electric power are indicated by thick lines with arrows.
In the conventional power plant configured as above, during traveling of the vehicle, the power from the engine is transmitted to the drive wheels, e.g. as follows: As shown in FIG. 51, the power from the engine is transmitted to the first ring gear, and is then combined with power transmitted, as described hereinafter, to the first sun gear. This combined power is transmitted to the second carrier via the first carrier. Further, in this case, electric power is generated by the second rotating machine, and the generated electric power is supplied to the first rotating machine via the controller. In accordance with the electric power generation, part of the combined power transmitted to the second carrier is distributed to the second sun gear and the second ring gear, and the remainder of the combined power is transmitted to the drive wheels. The power distributed to the second sun gear is transmitted to the second rotating machine, and the power distributed to the second ring gear is transmitted to the first sun gear via the first rotating machine. Furthermore, the power of the first rotating machine, generated in accordance with the above-described supply of the electric power, is transmitted to the first sun gear.
As described above, in the conventional power plant, power is recirculated via a path formed by the first carrier→the second carrier→the second ring gear→the first rotating machine→the first sun gear→the first carrier, and a path formed by the first carrier→the second carrier→the second sun gear→the second rotating machine→the first rotating machine→the first sun gear→the first carrier. This recirculation of the power causes very large combined power from the first ring gear and the first sun gear to pass through the first carrier and then pass through the second carrier as it is, so that to withstand the above large combined power, it is inevitable to increase the size of the first and second planetary gear units, which results in the increased size and manufacturing costs of the power plant. Further, in accordance with the increases in the size of the above power plant and the power passing through the power plant, losses generated in the power plant are increased to decrease the driving efficiency of the power plant.
The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide a power plant which is capable of reduce power passing through a distributing and combining device, whereby it is possible to attain reduction of the size and manufacturing costs of the power plant and enhance the driving efficiency thereof.    [Patent Literature 1] U.S. Pat. No. 6,478,705